1. Field of the Invention
The present invention relates to a magnetic disk and a glass substrate for the magnetic disk.
2. Discussion of Background
Glass has been noticed as substrate for a magnetic disk suitable for high density recording since it is excellent in surface smoothness, hard, has a large deformation resistance and has little surface defect. On the other hand, there has been requirement to expand the recording area of the magnetic disk so as to increase storage capacity, and especially, to expand the recording area to the outer peripheral portion. The requirement is commonly applied not only to a glass substrate but also a conventional aluminum substrate.
The glass substrate for a magnetic disk is polished. In the conventional polishing method, the glass substrate has been treated, especially by fine polishing with a relatively soft suede type foamed polyurethane in order to minimize flaws on the surface and to control surface roughness. When the polishing was conducted, a polishing pad was deformed at the outer peripheral portion of the glass substrate, whereby the outer peripheral portion could not maintain a flat surface. In other words, the outer peripheral portion can not maintain a level surface which is extension from the inner peripheral portion or the central portion in the surface of the glass substrate, and a downwardly bent portion or an upwardly bent portion, which is called Roll-off, was caused in the surface of the glass substrate. When a glass substrate having a downwardly bent portion or an upwardly bent portion is formed, a magnetic disk with a magnetic layer also has such an undesirable shape corresponding to the glass plate. When such magnetic disk was used and a magnetic head was caused to fly above the magnetic disk, the magnetic head inclined at a downwardly bent portion or an upwardly bent portion, particularly at the outermost peripheral portion, and accordingly, a stable flying movement could not be obtained. In particular, when the degree of deflection was large, the magnetic head became in contact with or collided with the magnetic disk, and accordingly, it was impossible to use the outer or inner peripheral portions as the recording area.